Fuel-air mixture control for aircraft heaters



April 8, 1947. A. A. ARN HYM FUEL-AIR MIXTURE CONTROL FOR AIRORAFT HEATERS Filed May 11, 1942 "Ill I'm Patented Apr. 8, 1947 UNITED STATES PATENT OFFICE,

FUEL-AIR MIXTURE CONTROL FOR AIRCRAFT HEATERS Alhert A. Arn hym, Point Loma, Calif., assignor, by mesne assignments, to Solar Aircraft Company, San Diego,.Calif., a corporation of California 1 Application May 11, 1942, Serial No. 442,442,

2 Claims. ,(Cl. 158-28) This invention relates to the heating of air craft and particularly aircraft that operate.

through awide range of altitudes and speeds, such as military airplanes. r

A primary object of the invention is to provide a practicable heater that is effective at high al Still another object is to provide a combus-..

tion heater that operates efficiently at high altitudes without the use of a supercharger.

Another object is to provide a combustion heater of very high overall efficiency that is relatively safe under allconditions of. use and re quires the minimum amount of attention and control.

Briefly, I accomplish the foregoing objects by so proportioning a gas-burning combustion heater as to provide the desired amount of heat when suppliedwith combustion air of the density existent at the maximum altitude and then automatically regulating the air and fuel supply to maintain the proper mixture at lower altitudes. This is in marked contrast to previous attempts products of combustion to the compartment to be I to solve'the problem by designing the heater for normal operation at ground level and then adding superchargers and the like to compensate for the reduced density of the air at high altitudes.

Other more specific objects and features of the invention will become apparent from the following detailed description of certain specific embodiments of the invention, as illustrated in the drawing, in which I Fig. 1 is a schematic diagram of one embodiment of my invention. l

Fig. 2 is a detail sectional view of a pressureactuated control valve employed in the system of Fig. l

Fig. 3 is a detail sectional view showin aconstruction of regulator valve that can be employed in the system of Fig. 1.

One of the chief advantages of the system of Fig. 1 is that the products of combustion are mixed with the heated air and delivered to the passenger compartment or cockpit. An advam' tage of this construction is that the overall heating efficiency of the device is made substantially since all the heat generated is delivered to the passenger compartment. A further advantage is that the necessity for having eflicient heat-transfer equipment is eliminated, thereby permitting much more compact and lighter apparatus. l

The foregoing advantages of delivering all the heated are obvious, but such an arrangement has heretofore been considered not feasible because of the dangerof contamination of the air in the passenger compartment. I have discovered, however, that this danger is negligible when employing propane as a. fuel gas. The propane itself is non-poisonous and is not injurious to life,

even when substantial concentrations of it are present in the air being breathed. Furthermore,

the fire hazard is relatively slight because the limits of inflammable air-propane mixtures are very narrow and an inflammable mixture would not be likely to obtain in practice with my apparatus, even should the safety devices employed fail to prevent discharge of unignited propane into the passenger compartment.

In normal operation, I have found that the combustion in my apparatus is practically 100% complete, so that all of the carbon in the propane is converted into carbon dioxide and all of the hydrogen is converted into water.. The carbon dioxide does not accumulate in sufficient concen trations to have any objectionable effect on the persons breathing it. This is particularly true because of the large volume of clean ventilating air that is supplied to and mixed with the combustion products in a proportion of approximately 10 to 1, so that the carbon dioxide in the combustion products forms only a small percentage of the total mixture emitted from the heater. The water vapor produced is actually beneficial, since the humidity is normally excessively low at high altitudes of flight.

Referring to Fig. 1, a stove I00 therein disclosed comprises a fire box Hll, roughly cylindrical in shape, positioned within an outer casing 02, with an air space I03 between the. two which may conreceives air from an air scoop l2 positioned exbe Iormedat the upper end integrally with a r header I09 having a flange I I0 which sets on and .is secured to the top wall I I I of the stove, the wall" II I having a central aperture therein for the passage of the burner tube I0'I. An igniter coil, I I2 is positioned exterior of the burner tube I01 ad- Jacent a pilot aperture II3. One end of the coil H2 is anchored and electrically connected 11- The otherend extends through an insulated bushing Ill to the rectly to the burner tube I01.

interior of the burner tube, thence through the header I09 and out through an insulated bushing I I5 in the latter.

I find that smoother operation and more uniform heating of the fire box IN is obtained if the apertures I08 in the burner tube I01 are positioned helically. It is also-desirable to employ a baflie II'I just below the neck I I8 of the fire box to divert the products of combustion laterally into the air stream issuing therepast and thereby reduce the chances. of flame being projected through the opening I00 at the time of ignition of the gas. The baille I I! is preierably conical in shape to give it rigidity and permit making it of relatively thin metal which will heat quickly and tend to complete the combustion of any incompletely burned gases.

It will be understood that the discharge opening I06 of the stove may be connected to a conduit system or may feed into a diffusing struc ture, ii! desired.

The system of Fig. 1 employs a pair of pressure regulating valves 35' and 36'respectively, corresponding in general function and structure to the valve illustrated, in Fig. 3. This valve consists of a casing 80 having a flexible diaphragm BI therein, which divides the interior of the casing into two separate chambers, 82 and 83, respectively. The chamber 82 is vented to the atmosphere by a vent 84 so that the upper side of the diaphragm 8| is always exposed to atmospheric pressure. The lower chamber 83 has a permanently opened discharge passage 85 and an inlet passage 86 that is adapted to be closed by a valve 81 under the control of the diaphragm 8|. Thus the valve ("is pivotally supported by a pivot 88 for swinging movement toward and away from the inner orifice of the passage 86 and has secured thereto an arm 89 connected by a link 90 to the diaphragm 8|. A spring 9| is compressed between the upper side of the diaphragm and an adjusting plug 92 so that when the pressures in the 82 to'compress the sprin 9|, the diaphragm is i moved upward to close the valve 81. In operation, the valve is maintained in a partly opened 7 position to admit gas at a rate just suflicient to maintain the desired pressure in the chamber 83 and the outlet passage 85. The value of this pressure is of course determined by the position of adjustment of the plug 92., I

The valve 35' correspond exactly in structure,

despite any imperfections in the pressure regulation afiorded by the regulator 35'.

.As a matter of fact, it would be possible with the arrangement shown in Fig. 1 to completely eliminate the regulator 35 (connecting the pipe I8 directly to the pipe I6) and still maintain the proper mixture of air and propane for complete combustion. However, such a system would be less desirable than that disclosed, because the amount of heat produced would vary with variations in the pressure in the scoop I2 resulting from variations in speed and altitude of the aircraft. l

In a particular apparatus tested, in which-the pressure developed in the air scoop I2 varied between three and ten inches of .water above atmospheric pressure, the regulator valve 35'- was adjusted to deliver air at a pressure of two and one half inches of water, and the regulator valve 36 was adjusted to deliver propane'at a pressure of approximately eight inches of water.

A higher pressure is employed on the propane than on the .air because high pressure is available, and it permits the use of smaller conduits able size for use with the particular pressure the regulator valve 36 is adjusted to deliver, in order to provide the proper relative proportion of air and propane for complete combustion.

The valves 35 and 36 function to deliver air and propane to the combustion chamber at substantially gauged pressures, irrespective of the altitude of flight and the atmospheric pressure. As used herein, gauge pressure means the pressure differential above the ambient atmospheric pressure such as would be indicated by an ordinary pressure gauge having an element exposed on one side to the pressure to be gaugedand exposed on the other side to the ambient atmospheric pressure. As a result, the relative proportions of air and propane'delivered to the heater are maintained substantially constant under different atmospheric pressures corresponding to difierent altitudes.

Although the propane'can be delivered directly to the combustion chamber, it is desirable to deliver it to the pipe I9 which conveys combustion air into the heater. By introducing the propane into the air stream in front of the heater, a better mixture of the air and propane is obtained.

Several refinements are incorporated in the control system of Fig. 1. cludes a manually operated switch 33' having two contacts I2I and I22, a safety switch 225, a, thermostat switch I23 and a relay I24. When the switch 33' is in open position, as shown in Fig. 1,

Thus the system in- .5 the entire system is inactive. When'the switch is closed on contact I22 a circuit is completed from the battery 32' through the safety switch 225 (assuming it to be closed), the thermostatic switch I23 andthe relay I24 to the igniter coil I I2. If the temperature is such as to demand heat, the thermostatic switch I23 will be closed, and current will flow through the relay I24 and the igniter coil H2 in series, pulling the relay andheating the igniter coil. Pulling of the relay completes a circuit over the contacts of the relay from the battery to a solenoid switch 30, opening the latter and permitting the flow of propane from the tank 22' to the regulator valve 36 and thence through the pipe 2| and through the jet in the end thereof into the pipe I9. The regulator valve 36 may be adjusted to maintain a pressure in the pipe 2 I of approximately'eight inches of water above atmospheric pressure. This pressure is applied through a tube I25 to a pressureactuated valve I26.

Thus referring to Fig. 2, the valve I26 comprises a. casing I21 mounted on the pipe I6, and having a diaphragm I28 which divides the casinginto two compartments I29 and I30, respectively. The compartment I29 is connected to the tube I25 which maintains the pressure in it the same as that in the pipe 2I. vented into the conduit I6. A butterfly valve I 32 in the pipe I6 has an arm I33 connected by a link I34 (extending through a guide opening I34a in the wall of the compartment I30) to the diaphragm I28, and the diaphragm is normally maintained in the position shown in Fig; 2 by a spring I35, opposing the pressure in the gas chamber I29, in which position the valve I32 is closed. However, when the solenoid valve 30 in the propane line has been opened and the pressure in pipe 2| rises to its normal value, this pressure, acting on the diaphragm I28, produces a force that compressesthe spring I35 and moves the diaphragm to open the valve I32 and admit air to the heating system. If the air pressure below the valve I32 and in chamber I30 rises much above the pressure of the propane in chamber I29, it partly closes the valve I32, thereby tending to equalize the pressure of the air delivered to the system. However, the valve I26 does not-close completely as long as any appreciable pressure exists in the pipe 2 I, since the spring I35 is preferably made very weak for the particular purpose of delaying closure of the valve I32 until substantially all gas pressure has been dissipated in the pipe 2| following shut-down of the system by closure of the solenoid valve 30.

In a system of the type shown in Fig. 1, in

which the products of combustion are delivered into the passenger compartment, it is undesirable. to permit accumulations of unburned propane in the combustion chamber following shutdown of the system. Such accumulations are prevented by the control system described, in which the pressure-actuated valve I26 continues to admit air to maintain combustion in the stove following the closure of the solenoid valve 30' until substantially all pressure in the propane pipe 2| has been dissipated.

The solenoid valve 30' is actuated through the relay I24 connected in series with the igniter coil II2, to provide for the automaticv closure of the I valve 30 and the shutdown of the system should The compartment Iis shut down the system. However, it may be partly I Another advantage of the. control system of Fig.-

l, employing the pressure-actuated valve I26, is

the igniter coil II2 burn out. The thermostatically controlled heater 34' in the propane tank 22' is also supplied with energizing currentover the contacts of the relay I24 so that it is always deenergized when the system is shut-down.

If it is desired to maintain the heater in operation at all temperatures and without automatic regulation, the switch 33' is closed on the contact |2| to complete a circuit from the battery 32 around the thermostatic switch I23.

It is often desirable to provide a visual signal to indicate whether or not the heating system is in operation. Such a signal may be had by a 7 I32. The lamp I31 will therefore remain lighted as long as gas and air'are supplied to the stove.

I have found that it is feasible to regulate the output of the stove to a certain extent by restricting the flow of the mixture of air and propane to the header I09. To this end a valve I40 may be inserted in the pipe I9 just in front of the header I09. This valve will never be completely closed, since it would not be employed to closed to reduce the output, ,when desired.-

As previously indicated, the circuit'from the ungrounded terminal of the battery 32' to the manual]; controlled switch 33 is completed through a safety switch 225. This switch is re-- sponsive to air pressure in the conduit I6. Thus the switch comprises a fixed contact 226 and a movable contact 221, the latter having a pin 221a engaging a slot 228a in a piston rod 228 connected to a piston 229 in a cylinder 230 connected at one end to the conduit I6. The piston 229 is normally urged to the left by a compression spring 23I to shift the movable contact 221 clear of the contact 226 and open the circuit from the battery 32' to the switch 33', so that even if the latter switch is closed the system cannot operate. However, when the aircraft is in motion, to develop pressure in the conduit I6, the force exerted on the piston 229 is sufficient to overcome the force of the spring 23I and move the piston to the right, carrying the contact 221 onto the contact 226. The spring 23I is chosen of such stiffness, and the length of the contact 226 over which the contact 221 can move is of such length, that the switch is open whenever there is insuflicient air pressure in the conduit I6 to operate the system.

The main purpose of the safety switch 225 is to exclude any possibility of products of incomplete combustion, or unburned propane, or cold air,.

from entering the cabin served by the heater. If it were not for this switch 225, closure of the switch 33 at a time when there was insufficient pressure in the conduit I6 would result in a stream of unburned propane passing through the heater and. entering the cabin.

It is important, of course, that the intake scoop I2 be located in an area of positive pressure on the aircraft skin.

In the description heretofore, the fuel specifically mentioned has been propane, because this fuel has special advantages already pointed out. However, it is to be understood that the invention is in no sense limited to the use of propane, or

special fuel pump for delivering the gasoline under sufiicient pressure.

Although for the purpose of explaining the invention certain specific systems incorporating the invention have been described in considerable detail, many departures from the exact designs disclosed can be made while still utilizing theinvention, and the latter is, therefore, to be limited only to the extent set forth in the appended claims.

I claim:

1. A combustion heater for aircraft comprising:' means forming a combustion chamber having a discharge opening, means for delivering atmospheric air at a first variablepositive gauge pressure, a pressure reducing valve for supply-.

ing air from said first means to said combustion chamber at an approximately constant, positive .gauge pressure thatis below said first pressure,

even to the use of a fuel that is a 'gas at ordinary delivered by said first-mentioned pressure plying means to said combustion chamber for delivering air thereto at approximately constant gauge pressure, a source of fiuid'fuel under high pressure, and means including a pressure-'reducing regulator valve and means for delivering fuel"; from said tank to said combustion chamber, said fuel regulator valve containing a control diaphragm exposed on one side to the pressure of fuel issuing from said regulator valve, and means for exposing the other side to the pressure of air ing valve.

ALBERT A. ARNHYM.

. l REFERENCES CITED The following references are of record in the file of this patent:

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